There is an increasing need to remove particulates and harmful substances from an exhaust gas discharged from an internal combustion engine, a boiler or the like in consideration of influences on the environment. In particular, regulations on the removal of a particulate material (hereinafter referred to as “PM” sometimes) discharged from a diesel engine tend to be strengthened worldwide. The use of a honeycomb filter as a diesel particulate filter (hereinafter referred to as “DPF” sometimes) for collecting and removing the PM receives attention, and various systems are suggested. The above DPF has a structure in which a plurality of cells as fluid through channels having a square cross-sectional shape are usually defined by porous partition walls, and the cells are alternately plugged, whereby the porous partition walls defining the cells perform a filter function. Here, the cross-sectional shape is the shape of each cross section of the cells cut along a plane perpendicular to the longitudinal direction of the cells.
In the DPF, the exhaust gas or the like containing the particulates is caused to flow into the filter from the side of one end of the filter, and the particulates are filtered by the partition walls. Afterward, the purified gas is discharged from the side of the other end of the filter. However, there is a problem that when the exhaust gas flows into the filter, the particulates contained in the exhaust gas are deposited on the one end (the end of the side into which the exhaust gas flows) of the filter to clog the cells. This is a phenomenon easily caused in a case where the exhaust gas contains a large amount of particulates or in a cold district. When the cells are clogged in this manner, there is a problem that the pressure loss in the DPF rapidly increases. To suppress such cell clogging, it is suggested that each cross-sectional area of open cells (inflow-side cells) in the end of the filter on the exhaust gas inflow side should be different from that of open cells (outflow-side cells) in the other end of the filter (in the end on the exhaust gas outflow side). Here, the cross-sectional areas are the areas of the cross sections of the cells cut along the plane perpendicular to the longitudinal direction of the cells.
In the honeycomb filter having the square cross-sectional shapes of the cells, however, when each cross-sectional area of the inflow-side cells is different from that of the outflow-side cells, the thicknesses of the partition walls defining the cells disadvantageously become small to decrease the strength of the filter at a part of sections (hereinafter referred to as “the intersections” sometimes) where the partition walls intersect with one another. Therefore, when the PM is deposited on the DPF, post injection is performed to burn and remove the PM. However, in this case, there is a problem that a stress is concentrated on the thinned part of the intersections, and the filter easily breaks down. Here, the section where the partition walls intersect with each other is a portion belonging to both the intersecting partition walls in the cross section of the honeycomb filter cut along a plane perpendicular to the longitudinal direction of the filter. Examples of the portion include a square region of an intersecting portion of the above cross section in a case where the linearly extending partition walls having an equal thickness intersect with each other.
Moreover, a honeycomb filter including inflow-side cells having large cross-sectional areas and outflow-side cells having small cross-sectional areas is suggested, in which the cells having large cross-sectional areas have an octagonal cross-sectional shape obtained by linearly cutting off the corners of a square shape (e.g., see Patent Document 1).
When the cells have the octagonal cross-sectional shape, the problem that a part of the above intersections becomes thin can slightly be solved, but the problem of the decreased strength remains. Therefore, there is a demand for a honeycomb filter which solves the problem of the clogged cells and the problem of the decreased strength.
Patent Document 1: FR-2789327-A